Continuous-wave-modulation signals have a modulating function that is continuous in nature. If a modulating function that exhibits discontinuities, such as a chain of pulses, is subjected to band-limiting filtering to reduce the rate-of-change or "slew rate" in the filter response, the filter response can constitute continuous-wave-modulation insofar as used with a detector constructed in accordance with the invention. The actual form of modulation used in the input signal for such a detector can take a number of forms. Among the well-known ones are amplitude-modulation (AM), pulse-amplitude-modulation (PAM), phase-modulation (PM), frequency-modulation (FM), and combined phase-and frequency-modulation as employed, for example, in mobile communications.
A desire in an amplitude-modulation detector is to decode the input signal encoded in the amplitude-modulation signal as free as possible from unwanted residues of carrier, carrier harmonics and sidebands thereof. This is the case in detectors handling digital sampled data, since low-pass or band-reject filtering to suppress these residues is cumbersome if their amplitudes are not substantially smaller than the decoded input signal. This is also the case for AM detectors handling analog signals, since the filters often require inductors or resistor-capacitor-amplifier filter elements that are of substantial physical size. Conventional balanced AM detector designs which cancel carrier fundamental and odd harmonics are helpful. However, the even carrier harmonics and their sidebands which remain as residues of the balanced detection process are appreciably large, since their combined amplitudes are a substantial fraction of the amplitude of the amplitude-modulation signal being detected. The suppression of these residues, while at the same time avoiding baseband roll-off of the higher frequencies of the recovered modulating signal, requires low-pass filtering of considerable complexity, which filtering it is desired to avoid.
F. F. Yassa in his patent applications serial numbers 190,191 and 194,257 described analog and digital species of a new type of amplitude-modulation detector using a prediction method to recover a baseband output signal free of baseband roll-off and also relatively free of carrier harmonics and their sidebands. The detector is supplied a first amplitude-modulation signal, which describes an input signal, and an unmodulated carrier signal, which is the same frequency as a carrier used in the generation of said first amplitude-modulation signal. The unmodulated carrier signal is amplitude-modulated with a predicted input signal, thereby to generate a second amplitude-modulation signal. The first and second amplitude-modulation signals are linearly combined so as to cancel the correlated portions of them and thereby develop a suppressed-carrier third amplitude-modulation signal. The third amplitude-modulation signal is synchronously demodulated with the unmodulated carrier signal, thereby to generate an error signal. The error signal is combined with the predicted input signal to recover an output signal. The predicted input signal is continuously revised based on the current output signal and is preferably generated by delaying the output signal slightly in time.
As pointed out by F. F. Yassa in his patent application serial number 194,257, an amplitude-modulation detector of this general type is especially attractive for handling digitized amplitude-modulation signals, because complex digital filtering of the output signals is unnecessary. This is because the amplitude of the carrier harmonics and their sidebands in the output signal are a substantial fraction of the amplitude of the relatively small suppressed-carrier third amplitude-modulation signal only, rather than a substantial fraction of the amplitude of the relatively large first amplitude-modulation signal as in prior-art amplitude-modulation detectors. Furthermore, as pointed out by F. F. Yassa in his patent application serial number 190,191, an amplitude-modulation detector of the general type is attractive for handling analog amplitude-modulation signals, particularly in instances where the carrier is not far above baseband, since the need for multi-section filtering to obtain baseband signals free of phase-distortion and carrier residues is avoided.
Another new type of continuous-wave modulation detector, described for the first time herein, permits both the recovery of information encoded in in-phase sidebands of the carrier and the recovery of information encoded in quadrature-phase sidebands of the carrier, which carrier may be partially or fully suppressed. The information is recovered substantially free of residues of carrier, carrier harmonics and sidebands thereof. An input signal has a real portion, which is encoded in amplitude-modulation form in the in-phase sidebands of a carrier, and has an imaginary portion, which is encoded in amplitude-modulation form in the quadrature sidebands of the carrier. This first complex amplitude-modulation signal is supplied to the detector together with in-phase and quadrature components of a complex unmodulated carrier signal. Means are provided for amplitude-modulating the in-phase component of the complex unmodulated carrier signal with the real component of a complex predicted input signal, and means are provided for amplitude-modulating the quadrature component of the complex unmodulated carrier signal with the imaginary component of the complex predicted input signal. The resulting orthogonal amplitude-modulation sidebands are linearly combined to generate a second complex amplitude-modulation signal. The first and second complex amplitude-modulation signals are linearly combined so as to cancel the correlated portions of them and to develop a suppressed-carrier third complex amplitude-modulation signal descriptive of their lack of correlation. This third complex amplitude-modulation signal is synchronously demodulated with the in-phase and quadrature components of the complex unmodulated carrier to detect the real and the imaginary components of a complex error signal. The real components of the complex error signal and of the complex predicted input signal are combined to recover the real component of a complex output signal, from which the real component of the predicted input signal is derived. The imaginary components of the complex error signal and of the complex predicted input signal are combined to recover the imaginary component of the complex output signal, from which the imaginary component of the predicted input signal is derived.
The complex-modulation detector described in the previous paragraph can also be used to detect angle-modulation of a carrier wave for sufficiently low indices of modulation.